Automatic mode detection in a dual operating mode RFID tag

ABSTRACT

A multi-mode, preferably dual mode, radio frequency identification (RFID) tag is adapted for automatic detection of whether a RFID reader located within communication range of the RFID tag is transmitting a continuous wave (CW) or modulated wave types of RF signal, and accordingly, mandating a response from the tag in read-only (RO) mode or read/write (R/W) mode, respectively. The tag includes means for designating one of the RO and R/W operating modes as a default mode of the tag, and for switching the tag from its default mode to its other operating mode, and vice versa, according to a rule for determining the frequency of occurrence of a selected event related to signal type of the reader. A device-implemented method of this automatic detection, and a method of fabricating the tag, are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/139,681, filed on May 31, 2005, now U.S. Pat. No. 8,195,985, and ofcontinuation patent application Ser. No. 13/468,371, filed on May 10,2012.

FIELD OF THE INVENTION

The present invention relates generally to radio frequency (RF)communication devices, particularly radio frequency identification(RFID) tags designed and adapted to operate in either read-only (RO)mode or read/write (R/W) mode, i.e., dual operating mode RFID tags, and,more particularly, to methods of and devices for performing automaticmode detection in such tags to enable an appropriate readout of thetag's memory content.

BACKGROUND OF THE INVENTION

Typically, radio frequency identification (RFID) tags are secured on orin movable items. In certain instances the tag may be placed on acontainer for a multiplicity of the same items, rather than on the itemsthemselves. The identity of and perhaps other information relating tothe tagged item is stored in its tag, and is transmitted by the RFID tagto a remote RFID interrogator, or reader, in response to a scan, queryor command from a reader within the response range of the tag, i.e., arange suitable for RF communication between reader and tag. In itssimplest form, the conventional RFID tag consists of a transponder andan antenna. Sometimes, the RFID tag itself is referred to as atransponder.

RFID tags may be either passive or active. A passive RFID tag lacks aninternal self-sufficient power supply, e.g., a battery, and reliesinstead on the incoming RF beam (in this example, a query by the reader)to produce sufficient power in the tag's internal circuitry to enablethe tag to transmit a response. In essence, the query induces a smallelectrical current in the tag's internal antenna, which serves as thepower source that enables a reflected or backscattered response in aread-only mode. Accordingly, a passive RFID tag is generally limited inthe amount of data that can be furnished in its response, e.g., an IDnumber and perhaps a small amount of additional data. But the absence ofa battery leads to certain advantages, primarily that a passive tag canbe fabricated at much less cost and with considerably smaller size thanan active tag. Among other current uses, passive RFID tags mayeventually replace the ubiquitous universal product code (UPC), the barcode strip found on myriad products in the stream of commerce—theimprinted strip requiring a line of sight optical scan to produce theUPC readout and the resulting computerized display or printout of price(at a point of sale of the bar-coded product) and other informationregarding the product.

The on-board battery of an active RFID tag can give the tag a greaterresponse range, along with greater accuracy, reliability and datastorage capacity, but the active tag has the aforementioneddisadvantages of cost and size relative to the passive tag. The batteryitself is quite small by present day standards, but not enough toovercome the size disadvantage.

The principles of the present invention are equally applicable topassive and active RFID tags.

A typical conventional RFID tag reader employs a transceiver, a controlunit and an antenna for communicating with the tag at a designated RFfrequency among several allocated for this purpose. An additionalinterface such as RS 232, RS 485, or other, may be provided with thereader to allow data received from the tag to be forwarded to anothersystem.

Historically, the RFID tag has constituted a read-only (RO) device,capable of transmitting only fixed, invariable information stored in thetag memory of the integrated circuit chip in which the tag isfabricated, as backscatter readout when the tag is scanned by the readerin an RF communication between reader and tag. More recently, read/write(R/W) RFID tags have been developed, which are adapted to be programmedor altered in memory content by write data received by the tag in thereader's RF beam. Data is stored in the tag memory such as anelectrically-erasable programmable read-only memory, or EEPROM, and mayconsist of original data, an overwrite of previously stored data,refreshed data, and/or entirely new data, which is then available forreadout from the R/W RFID tag upon the tag's receipt of a scan (in thisexample, a command) from a RFID reader.

A RFID reader transmits a continuous wave (CW, i.e., non-modulating)scan at the designated RF communication frequency to interrogate aremote RFID tag, and in response, receives an automatic backscatter RFsignal transmitted by the tag, containing RO data stored in the tag'smemory. Communication with a R/W RFID tag requires a RFID reader thattransmits a modulated RF signal (in contrast to the CW RF signalgenerated by a reader for acquiring RO data from a tag) by which data isprovided to the tag for programming (overwriting) or to initiate otherfunctions of the tag.

A RFID tag that is implemented according to the present invention toperform in either mode of operation, RO or R/W, according to whether itreceives a CW or modulated incoming RF command signal from the reader,is termed herein as a dual mode tag. The dual mode tag may be equippedwith an EEPROM device to serve both the read-only operation and theread/write operation. As used in this specification, the term“read-only” refers not to the type of memory, but rather, in the RFIDsense of a tag that operates, when it is powered up, to send the samedata continuously and repeatedly. In the case of a tag that operatesonly in the RO mode, it does nothing else but to perform this repeatedtransmission. In contrast, the term “dual mode” refers to a tag'sability to operate in an agile way to sense whether the reader isinterrogating or commanding with a CW signal or a modulated signal, andto respond accordingly with read-only operation or with read/writeoperation, whichever is appropriate.

A dual mode RFID tag is desirable in situations where the same tag is ormay be subjected to two or more different applications in the normalcourse of use or exposure of the item, object or product that bears thetag. For example, a vehicle may be provided with a RFID-tag to allow thevehicle to be machine-recognized as being authorized to participate in asystem for automatic collection (as by debiting) of a toll upon thevehicle passing a scanning point at which a remote reader is located,such as at a toll collection station along a highway or an entry pointin a parking garage or lot. Or the RFID tag may identify the vehicle tothe remote reader as being authorized to enter the grounds of a securefacility or area outfitted with access control gates, so as toautomatically raise the gate as the vehicle approaches the gated entrypoint and is so identified. Other system applications abound, includingthose in which RFID tagged badges are issued to be worn by employeesauthorized by various levels of access within a secure facility. And aparticular system may be used in several different locations or inseveral different applications. With the passage of time, the particularsystem may be upgraded, or planned by its owner or operator to migrate,from a simple read-only technology to a somewhat more complex read/writetechnology.

In such circumstances, a period of time exists during which RFID readersmust be provided to read both legacy (originally issued) RO tags andnewly issued R/W tags. A difficulty arises because the newly issued R/Wtags will not operate with some or even many of the multiplicity ofroadway, parking, airport and garage applications with which the oldertags had been designed to operate, unless all of the applications areupgraded concurrently. And the older tags may be inoperable in theupgraded systems or applications. In these instances it becomesessential to provide tags that are configured to operate in both modes,RO and R/W, to enable usage and proper system performance in all ofthese applications.

In another application where a dual mode RFID tag is advantageous, amulti-level or multi-step system may be used for enabling or denyingaccess to different areas of a secure facility. The facility may haveone level of security for individuals to gain access to an outer portionof the facility, and a higher level of security for access to an innerportion of the facility. At entrances to the lower level of secureaccess, RFID readers may be used to perform a simple CW scan of employeebadges containing a RFID tag, which produces a RO response to allowadmission past the security stations. The badges of employees who havebeen granted the higher security level, however, may utilize dual modetags that provide not only the RO response at the lower level entrances,but an additional appropriate response to R/W readers stationed at oneor more entrances to the higher security portion of the facility.

Thus, at least two distinct advantages exist for providing a dual modeRFID tag, in which the operating modes are both a CW (RO) response and amodulated signal (R/W) response. One of these advantages is the capacityfor interoperability between independent applications, and the other isthe provision of a distinct migration path from one response type to theother for independent applications.

Prior mode detection schemes for dual mode RFID tags are inefficient andslow to respond, especially to support moderate to high speedapplications of the tags and readers, and therefore have been found tolower system margin. System margin is tantamount to a figure of meritbased upon the number of times a given type of transaction can becompleted or the number of times a given set of frames can be read in agiven time interval. A typical frame of interest has a length of 128bits of data, and includes appropriate frame markers. The greater thenumber of transactions that can be accommodated in the given timeinterval, the higher the system margin. High system margin is especiallyimportant in moderate to high speed dynamic applications such as “on thefly” toll tracking and collection systems that rely on RFID tag readingas vehicles pass at or near highway speed through a designated unmannedtoll collection lane or lanes of a toll plaza.

It would be desirable to detect and comply with a command in an RFsignal from a remote reader to a dual mode RFID tag by a method andmeans of automatic mode detection, and where appropriate, consequentmode switching or mode assumption that optimizes the number oftransactions that can be handled in a given time period.

As used in this patent specification, the term “mode detection” isintended to refer to recognition by a dual mode RFID tag of a directive,command or instruction contained in an incident RF signal for the tag torespond in an operating mode consistent with such command; and the term“automatic mode detection” is intended to refer to a tag performing suchmode detection, and when applicable, to undergo mode switching from oneto the other when it is not already in the proper mode called for by thereader's command, or to undergo mode assumption, promptly and withoutneed for any type of manual intervention. The term “mode assumption” isintended to mean a dual mode RFID tag commencing the correct operatingmode from a condition in which the tag is idle, inactive, or shut off,but in readiness to receive the command as an incoming RF signal from anearby RFID reader. And the terms “mode switching,” and “mode change,”are used interchangeably herein and intended to refer to a dual modeRFID tag undergoing a change in operating mode from RO to RW, or viceversa, regardless of how the switch or change is manifested.

SUMMARY OF THE INVENTION

It is a principal object of the present invention to provide a dual modeRFID tag having the ability and agility to detect and promptly assumethe operating mode mandated by the RF signal transmitted by a reader incommunication range of the tag, according to whether the reader is CW(calling for response in RO mode) or modulated (calling for response inR/W mode). A related object is to provide a dual mode RFID tag adaptedto undergo prompt mode switching according to a rule that determines thefrequency of occurrence of an event related to whether the RF is CW ormodulated. Still another object of the invention is to provide such adual mode RFID tag to enable an optimum number of transactions to beconducted or handled by the RFID system in a minimum amount of time.

According to an aspect of the invention, a device-implemented method ofautomatic mode detection is performed by a dual mode RFID tag adapted tooperate in either of a RO mode or a R/W mode. The device-implementedmethod automatically detects the operating mode asserted by the RFsignal from a RFID reader located within communication range of the dualmode RFID tag according to whether the reader transmits a CW type signalor a modulated type signal. “Asserted” refers to the mode in which thereader whose RF signal is detected by the tag is seeking or mandating aresponse by the tag, according to the reader's signal type (CW ormodulating). The detection then evokes a response from the dual modeRFID tag in the asserted mode. The method includes the steps of assuminga programmed dominant one of the RO and R/W operating modes, and if thedominant mode is not the asserted mode, switching to the other operatingmode according to a rule implemented at least in part by criteria todetermine the frequency of occurrence of a selected event related to thesignal type of the reader over a designated time interval. The switchingrule may be implemented in further part by criteria to compare the levelof the RF signal from a reader detected by the tag to a threshold level.

According to another aspect of the invention, the dual mode RFID tagincludes circuit means for causing the tag to assume a programmeddominant one of the RO and R/W operating modes. If the dominant mode isnot the mode asserted by the reader's RF signal, the circuit meansswitches the tag to the other operating mode according to criteriaimposed by the rule referred to in the immediately preceding paragraph.

Preferably, the selected event whose frequency is to be determined isthe falling edge of a waveform derived from the reader's RF signaldetected by the tag.

According to a feature of the present invention, the dominant operatingmode is the default mode of the tag, and the circuit means causes thetag to revert to the default mode each time the RF signal from a readerthat asserted the other operating mode is no longer detected by the tag.

According to another feature of the invention, the switching rule isimplemented to establish a degree of reluctance that must be overcomefor the tag to switch from its default mode to its other operating modewhen asserted by an RF signal from a reader. The switching rule may alsobe implemented to establish a degree of acceptance of the tagconsiderably less stringent than the degree of reluctance, to switchfrom the other operating mode back to the default mode when asserted byan RF signal from a reader,

According to another aspect of the invention, a method is provided forfabricating a dual mode RFID tag adapted to operate in either a RO modeor a R/W mode for automatically detecting the operating mode mandated bythe RF signal from a RFID reader located within communication range ofthe dual mode RFID tag, wherein the reader is either CW or modulated, soas to evoke a response from the tag in the mandated mode. The methodincludes the step of implementing means to adopt one of the RO and R/Woperating modes as a default mode of the tag, and if the default mode isnot the asserted mode, to switch the tag to the other operating modeaccording to a rule that designates an event related to signal type ofthe reader and determines the frequency of occurrence of the designatedevent.

Still another aspect of the invention resides in dual mode RFID tagadapted to operate in either RO mode or R/W mode, for automaticallydetecting the operating mode mandated by the RF signal from a RFIDreader located within communication range of the tag, whether the RFsignal is transmitted by a CW reader or by a modulated reader, so as toevoke a concomitant response from the tag. The tag includes means fordesignating one of the RO and R/W operating modes as a default mode ofthe tag, and for switching the tag from its default mode to its otheroperating mode according to a rule that determines the frequency ofoccurrence of a selected event related to whether the reader is CW ormodulated.

Therefore, another object is to provide a method of automatic detectionof the proper operating mode to be assumed by a dual mode RFID tagassociated with an item of commerce, for response to commands containedin readers' RF signals, including establishing detection rules for eachof different types of commands, so the tag will provide a response modethat satisfies the established detection rules.

Yet another object is to provide a RFID tag having a logic circuit thatenables rapid switching of the tag between its dual operating modes torespond accordingly to either of the different types of RF signal scanfrom CW and modulated readers, to accommodate high speed applications.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and still further objects, aims, features, aspects andattendant advantages of the invention will become clear to those skilledin the art from a consideration of the following detailed description ofthe best mode presently contemplated for carrying out the principles ofthe invention, including alternative embodiments and implementations,taken in conjunction with the accompanying Figures of drawing, in which:

FIG. 1, comprised of 1A and 1B, is a circuit diagram of an automaticmode detection and mode switching circuit for a dual mode RFID tag, inaccordance with a presently preferred embodiment and method of theinvention;

FIGS. 2A, B, C and D are waveform diagrams at various stages ofprocessing of an exemplary RF signal used to trigger operation of thecircuit of FIG. 1 for automatic mode detection and mode switching;

FIG. 3 is a flow chart illustrating operation of the circuit of FIG. 1with the waveforms of FIGS. 2A-D, to achieve the mode detection andautomatic mode switching of the invention; and

FIG. 4 is an executive state diagram that controls the execution of eachoperating mode.

DETAILED DESCRIPTION OF A PRESENTLY CONTEMPLATED Best Mode of Practicingthe Invention

In describing a preferred embodiment of the invention illustrated in thedrawings, certain specific terminology will be used for the sake ofclarity. However, the invention is not intended to be limited to thatspecific terminology, and it is to be understood that the terminologyincludes all technical equivalents that operate in a similar manner toaccomplish the same or similar result.

Referring now to FIG. 1, a logic circuit is implemented in integratedcircuit (IC) form, and in particular, as an application specificintegrated circuit (ASIC) 10, either within the IC chip in which thedual mode RFID tag is fabricated, or on a separate chip electricallyconnected to the IC chip of the RFID tag. If the RFID tag is passive,logic circuit 10 is powered in the same way as the typical transponderis powered; that is, by the DC voltage that is obtained from conversionof the RF beam incident on the tag's antenna from an interrogating RFIDreader. If the tag is active, it is implemented with a battery for itsinternal power supply. Logic circuit 10 is implemented in its ASICarchitecture to perform automatic mode detection and to assure rapidcompliance of mode selection therewith by the RFID tag, so as to renderthe tag highly agile in responding appropriately to the type of RFsignal received from the RFID reader, either CW (calling for a ROresponse from the tag) or modulated (calling for a R/W response from thetag).

The ASIC logic circuit 10 is connected between the tag's antenna 12 andthe input to its transponder. The front end of the ASIC includes fieldeffect transistors (FETs) 14, 15, the latter supporting a backscatterresponse to a scan of the RFID tag, whether active or passive. The logiccircuit also comprises, among other components, various AND (or NAND)gates 16, 17, 18, 19 and 23, OR gate 20, a read-only protocol encoder 21and read/write protocol encoder 22, and their respective inputs andoutputs. The ASICs front end is implemented to obtain digital signalinformation from the RF input signal received at antenna 12 and todetect whether the signal is modulated or CW. If a modulated signal isdetected, it is subjected to further processing by logic circuit 10including the operation of a data detector 25 and an edge detector 27.

RO encoder 21 is labeled “ATA” in the presently preferred embodiment ofFIG. 1, which is merely an internal designation for a protocol that,when activated, continuously scrolls 128 bits of specific data at aspecific rate and in a specific format for the RO response of the RFIDtag. This is referred to as RO protocol, or simply as “read-only” or“RO,” hereinafter. Similarly, R/W encoder is given the internal label“SeGo” in the Figure, which is an internal designation for a protocolthat, when activated, produces the R/W format for the R/W response ofthe tag, and will be referred to hereinafter, except occasionally, bythe terminology “read/write” or “RW”.

Turning for the moment also to FIG. 2, the envelope of an exemplary RFsignal received by the tag from a modulated RFID reader is illustratedby the waveform in part A of the Figure. Modulation from the tag's ASIC10 is exemplified by the waveform of part B of FIG. 2. The resultantoutput waveform from data detector 25 is illustrated in part C of theFigure. The falling edges of the data detector's output waveform,illustrated in part D of FIG. 2, are detected by edge detector 27.

Referring again as well to FIG. 1, certain parameters, or variables, areused to configure the logic circuit 10, and by extension, the RFID tagitself. These configuration variables include a Dominant Bit, andnumbers M (or m), N (or n), Min, and Max, whose values are set to assistin establishing rules within the tag for automatic detection of theoperating mode dictated by the incoming RF signal (“automatic modedetection”). The Dominant Bit (“dom_mode”) is a configuration variable(either “0” or “1”) applied to multiple AND gates 29, 30 and 31 (in thelatter case as “not(dom_mode)”), as well as to edge count comparator 28and M/N multiplexer (M/N mux) 33. “Min” and “Max” are configurationvariables applied to edge count comparator 28; “m” (or “M”) and “n” (or“N”) are configuration variables (“m_reg” and “n_reg”, respectively)applied to M/N mux 33. The “m_n” output of the multiplexer 33 isdelivered to M/N comparator 34, which, depending on that input and anadditional input from a temporary count latch, may generate a“change_state” (or “change mode”, being the same) command to the tag'stransponder.

A Dominant Bit of “1” configures the dual mode RFID tag with theread-only (RO) operating mode as its dominant state or mode, and aDominant Bit of “0” configures the tag with the read/write (R/W)operating mode being dominant. Of course, this assignment of the valueof the Dominant Bit is completely arbitrary and is not a limitation onthe invention—the value “1” might just as easily have been used fordefining the R/W mode, and the value “0” used to define the RO mode. Itis to be anticipated that certain dual mode tags will encounter morereaders of one type (e.g., CW, asserting a desired response by the tagin RO mode) than of the other type (e.g., modulating, asserting adesired response by the tag in R/W mode) when the tag is scanned. Andsome new tagged products with tags operating in the R/W mode may needdominant settings to assure performance levels achieved by older taggedproducts. The ASIC 10 can operate only with the “dom_mode” set one wayor the other, and the value of this bit defines the ASIC's defaultoperating mode. Hence, upon completing a boot-up, the tag will commenceoperating in that default state, whether it is the RO mode or the R/Wmode. The “dom_mode” bit also sets an “acceptance” or a “reluctance”factor when the tag is switched from one mode to the other.

For example, if the “dom_mode” bit is set for RO dominance (causing thetag to operate by default in its RO operating mode), then the rule forchanging or switching the tag to the R/W operating mode will bestringent or “reluctant.” Reluctance indicates a greater degree ofdifficulty imposed by criteria implemented according to the rule forswitching operating modes, i.e., a reluctance of the state machine toswitch, from its preset dominant operating mode to its other mode. Thisreluctance is manifested by a need for several hurdles imposed by suchcriteria to be overcome before the tag can switch modes from RO to R/W(in the context of this example, but a similar situation applies if R/Wwere the dominant mode, and a switch to the RO mode is asserted by theincoming RF signal from the reader).

Continuing with this example, according to the invention the criteriaimposed by the “reluctance” rule to be overcome for the tag to undergomode switching from its dominant operating mode to its non-dominant (or“recessive”) operating mode, are as follows: The incoming RF signal must(1) exceed a factory-preset R/W RF threshold (although factorypresetting is preferred for the embodiment under discussion here, thatthreshold could be set in the field if desired), (2) result in thedetection of a number of falling edges in the signal that lies betweenthe numbers assigned at the outset to the “Min” and “Max” configurationvariables, and (3) result in that detection in “N”.times.200-microsecond(200 .mu.sec, or 200 M*10) windows, where “N” is a pre-assigned numberfor this configuration variable.

In other words, the rule for switching from the dominant, or defaultmode, to the non-dominant mode is implemented, at least in part,according to the frequency of occurrence of an event of interest relatedto the reader's signal type. In this particular case, the event is afalling edge of a waveform derived from the reader's RF signal detectedby the tag, over a designated time interval. And to a lesser extent,this stringent or reluctant rule is implemented in further part by acomparison of the level of that RF signal to a specified thresholdlevel.

After the dual mode RFID tag is switched to its non-dominant operatingstate (R/W in this example), the rule for changing or switching the tagback to its dominant read-only mode (prompted in this example by the tagencountering—i.e., being scanned or commanded by—a CW reader) shifts toa much less rigorous or stringent one, arbitrarily termed “acceptance.”Acceptance is manifested as a considerably lesser degree of difficultyfor returning the tag to its dominant (default) operating mode (i.e., abias toward accepting a change from the non-dominant to the dominantmode), compared to the “height” of the barrier established by criteriaimposed by the “reluctant” rule to switch the tag from its dominant toits non-dominant operating mode.

The “acceptance” rule is met (to cause a return to the RO state as thedominant mode in this example) if, over a prescribed period of timedefined by (10.times.“M”) 200 .mu.sec windows, the tag detects a numberof falling edges of the processed incoming RF signal less than thenumber assigned to the variable “min.” And it goes without saying thatvery few, if any, edges would be expected to be detected in theprocessed incoming RF signal, since in this specific example, the RFIDreader encountered by the tag is, by definition (subject to the caveatbelow), CW. A further albeit lesser determination made by the“acceptance” rule resides in a comparison of the level of the reader'sRF signal to a specified threshold level, but in the particular case ofthe RO mode being the default mode, reversion to that mode will occurirrespective of whether or not that detected signal level exceeds thethreshold. In fact, the mere disappearance of the previously detectedmodulating signal from a reader would cause a return of the tag to itsdefault RO mode.

Similarly, if the ASIC is configured (by the setting of the appropriateconfiguration variables) for dominance of the R/W mode, the rule forchanging to the RO mode becomes “reluctance.” To overcome thisreluctance, (1) the incoming RF signal should exceed or at least meet aRO threshold (preferably also preset at the factory for the embodimentof this example, but alternatively, may be set in the field), and (2)the number of falling edges detected in the resultant output waveformfrom data detector 25 of ASIC 10 (as represented by part D of FIG. 2)must be less than the number assigned to the configuration variable“Max” for (10.times.“M”) consecutive 200 .mu.sec windows, where “M” isthe pre-assigned number for this configuration variable.

Here, the number assigned to configuration variable “M” is used,together with a “times 10” multiplier produced by operation of a divider32 between Edge Count Comparator 28 and Temp Count Counter 35 (FIG. 1),to define the number of consecutive 200 .mu.sec windows in whichdetection of less than the “Max” number of edges must occur before theASIC logic circuit 10 will mandate mode switching of the tag from itsdominant R/W mode to the non-dominant RO mode. Once in the non-dominantRO state, the rule for switching back to the dominant R/W mode becomes“acceptance.”

To determine whether the “reluctance” rule has been met (and similarlyfor the “acceptance” rule), two configuration variables that control thenumber of 200 .mu.sec windows need to be set. In this example ofdetermining whether the reluctance rule is met, these variables orparameters are “M” and “Max”. Selection (programming) of the variablesis undertaken with a view toward developing a formula that makes theswitch from the dominant (default) operating mode to the other operatingmode considerably more difficult to achieve than to switch from thelatter mode to the dominant mode. The formula inquires into the numberof times an event occurs within a selected period of time, where theevent is made relevant by the nature or type of the reader. For thepresently preferred embodiment, that event is the existence of fallingedges in the logic circuit-processed RF signal input to the tag from thereader. In addition, a preset or programmed threshold for the incomingRF signal also may play a role, albeit one of lesser significance,because the desire with respect to that aspect is to assure that theincoming signal is not signal interference from a distant source ormerely noise. The RO (CW) threshold is of least interest because it ishighly unlikely that either interference or noise will mimic acontinuous wave signal.

The “acceptance” and “reluctance” rules, whichever may be applicable toa particular situation, are determined as follows. If the Dominant Bit=1(i.e., RO mode dominant, which becomes the default mode of the tag), andthe reader whose RF signal is detected is modulating, the rule for thetag to be switched to the R/W mode is stringent, i.e., reluctant. Let itbe assumed that the acceptance rule to produce a switch from R/W modeback to the default RO mode is 10.times.M.times.200 .mu.sec where M is a6-bit value in the range of 0<10.times.M.times.200 .mu.sec<126 msec.Conversely, if the Dominant Bit=0 (R/W mode made dominant, or thedefault mode), the reluctance rule for switching from R/W mode to thenon-dominant RO mode might be 10.times.M.times.200 .mu.sec, where M is a5-hit value in the range of 0<10.times.M.times.200 .mu.sec<62 msec. Thevalues 126 msec and 62 msec pertain to the exemplary dwell time of thereader for processing data, as explained below. The point is that thesevalues are matched to the system application, i.e., specific tagattributes.

In these two cases the acceptance factor and the reluctance factor areswapped, or reversed, depending upon which of the two modes is thedominant one. The difference in the number of bits used for the variableM is that in the second example, it is assumed that one bit is neededfor another purpose and therefore sacrificed. This is acceptable. The Mvalue is limited based upon logic gate resources, e.g., the larger the Mvalue the more flip-flops are needed for the logic circuit. It isdesirable, but not essential, to limit the number of logic elements tothe minimum necessary to reasonably accomplish the task, for the purposeof reducing both size and cost of the tag.

Most RFID readers have short periods of time (dwell time) to processdata after a command (an interrogation) is sent to activate a tag.Although dwell time varies from reader to reader, it is generally lessthan 10 msec. It is possible that a reader might have a dwell timelarger than 126 msec, but in that circumstance, the application in whichthe tag is to be used would not be high speed. In this situation, thedwell time may surpass the m (or M) period defined by the formula in theimmediately preceding paragraph, possibly making it necessary for thetag to be switched from the RO state before another R/W transaction canoccur. Such a change would be necessary only if the dwell time of thereader exceeded m and the tag had moved from R/W mode to RO mode.

The variable N defines the number of consecutive 200 .mu.sec windowsover which the Min and Max conditions must be met before the ASIC willswitch modes, i.e., RO to R/W (but not vice-versa). The variable Nestablishes a key part of the “acceptance” rule or the “reluctance”rule, whichever may be applicable, and allows the ASIC 10 to be matchedto the system application, and thereby, specific reader attributes. Ifthe Dominant Bit=1 (RO mode dominant), the reluctance factor isN.times.200 .mu.sec. By way of example, N is a 6-bit value, and0<N.times.200 .mu.sec<12.6 msec. If the Dominant Bit=0 (R/W modedominant), the acceptance factor is N.times.200 .mu.sec. For thisexample, N is a 5-bit value, and 0<N.times.200 .mu.sec<6.2 ms.

The variable Min defines the minimum number of transitions (fallingedges) needed or allowed in each 200 .mu.sec window before incrementingTemp_Count 35. Temp_Count is satisfied when it reaches N or M. The Minvariable allows the ASIC to be matched to the systemapplication/specific reader attributes, the RF footprint (which is thefield pattern of the RF signal from the reader at the tag antenna)characteristics (e.g., nulls, rapid changes in field strength, and soforth), and interfering AM (amplitude modulation) sources.

The variable Max defines the maximum number of transitions (fallingedges) allowed in each 200 .mu.sec window before incrementing theTemp_Count. This variable similarly allows the ASIC to be matched to thesystem application/specific reader attributes, RF footprintcharacteristics, and interfering AM sources.

With reference again to FIGS. 1 and 2, the output waveform of datadetector 25 is applied, together with edge_counter_input_enable, to NANDgate 36, which performs a logical multiplication to provide an edge_inclock to an edge-triggered D flip-flop 37. The negative edge-triggeringis achieved by having a NAND gate in the circuit. Thus, flip-flop 37changes state at the rising edge (negative edge out of the data detector25) of each edge_in clock pulse, and is sensitive to its input at D onlyat this transition of the clock.

In the edge-triggered D type of flip-flop, a single bit of data is to bestored, if applicable, at each transition of the clock. The flip-flop 37stores a 1 in the set state (this occurring when a high is on the Dinput at the time of the clock transition), and a 0 in the reset state(this occurring when a low is on the D input at the time of the clocktransition). Output Q follows D at the clock edge, and is the input D toeach of a pair of edge-triggered flip-flops 38, 39, with oso_ref andoso_ref not (the positive and negative phases of an internal clock),respectively. The Q output of each of the latter flip-flops is appliedto OR gate 40, which performs logical addition of the two as an input toedge counter 42. The output of the logic circuit 10, at M/N comparator34 is an instruction to the tag to change state (switch operating mode)based in part on the RF level of the input signal detected at antenna 12and in further part on the respective values of the configurationvariables that determine the logic circuit configuration as used in theestablishment of the mode detection/switching rules.

The following summary of operation of a dual mode RFID tag implementedaccording to the principles of the present invention is presented by wayof illustration and not limitation of the invention. The tag is adapted,by definition, for both RO and R/W operation, but has its ASIC 10configured for a dominant operating mode of either RO (by programming orotherwise setting its dominant bit=1) or R/W (dominant bit=0). The otherconfiguration variables Min, Max, N, and M may be programmed torespective values according to the rigidity or looseness (or somethingbetween the two) of the mode detection rules depending upon theapplication in which the tag is to be used and/or as desired by theuser. In general, according to the invention, these rules will beestablished to make it easier (acceptant) for a tag to revert to itsdominant mode, and harder (reluctant) to be switched to its recessivemode.

The remote RFID tag reader is either implemented (i) to transmit acontinuous wave RF signal (i.e., a CW reader) to evoke a backscatterresponse of continuous, repetitive data from memory (e.g., EEPROM) inthe tag, or (ii) to transmit an RF signal with modulation (i.e., amodulating reader) to selectively write selected data into and/or readselected data from the tag's memory, subject in either case to thereader being located within the response range of the tag for therespective signal. Typically, the reader has a fixed position and thetag (specifically, the product or article to which it is secured) ismovable, but the arrangement for a given application may be vice versa,or both reader and tag may undergo movement. While it is anticipatedthat the tag will be subjected to separate queries by both CW readersand modulating readers, it is conceivable that a single reader may beimplemented for selective transmission of CW or modulating signals, toread data in either operating mode of the tag.

Referring, for purposes of this illustrative summary of tag operation,to the flow diagram of FIG. 3, when the tag is activated (according towhether the tag is passive or active) the ASIC 10 is also booted up (at50) with the selected values of the configuration variables (at 52), andthe tag assumes its default mode (the selected dominant mode ofoperation), either RO or R/W as the case may be (at 54).

If the dominant mode is R/W (at 56), that operating mode is entered andthe R/W state machine moves to the IDLE state. The IDLE state is definedas a span in which the R/W state machine is awaiting an R/W command, andas being the state that edges (or the lack thereof) are detected, usedfor determining whether a switch of modes is necessary. The edgesdetected must be less than max for an interval of ten times thedesignated value of the configuration variable M (i.e., a total ofM.times.10) consecutive 200 .mu.sec windows before a switch to the ROmode can occur. The ASIC hoots up and moves directly to this mode ofoperation when R/W mode dominant is designated by selection of thedominant bit=0.

The ASIC monitors any RF signal received as an input to the tag at itsantenna to detect a query from either a CW reader or a modulatingreader. When modulation is detected as a result of the R/W detectionrule having been met (at 58), the ASIC continually processes R/Wcommands (at 56, 58 and 60), but it should be noted that actually, theidle state is included in every R/W command. The processing includes thewriting (including overwriting of previously stored) and/or reading ofdata in the tag's memory according to the detected RF input signal,until no further modulation is detected during the IDLE state of theASIC. It is deemed that no modulation is detected at the RF input fromthe tag antenna when a number of detected edges<Max have occurred inM.times.10 200 .mu.sec consecutive windows while the RO threshold isexceeded by the average RF level.

If this “no modulation detected” condition is satisfied, the reluctancethreshold is overcome and the ASIC thereupon switches the tag to RO mode(at 66) and commences continuous scrolling of the RO frame data (at 68).The backscattering FET 15 (FIG. 1) is enabled (at 72) since the averageRF signal level is above the read-only threshold (at 70) (according tothe condition of “no modulation detected”), so a response is sent by thetag to the CW reader. The tag scrolls its RO frame until the switchingcriteria to resum R/W operation is satisfied or a loss of power occurs.It should be noted that the ASIC, even though designed for dual modeoperation, might be configured RO mode only. In the dual modeconfiguration, the windows are always open to allow detection of atransmission from a reader. Thus, while the tag is activated, the ASICis continuously monitoring (searching) for RF signals received at thetag's antenna to detect a condition of modulation or of no modulationand thereby cause the tag to switch to its other mode (here, R/W), or tocontinue in its then-current mode (here, RO), accordingly.

Once in the RO mode, either of two distinct conditions can cause theASIC to return the tag to the R/W mode, viz.: (i) modulation is detected(at 62), or (ii) the average RF level falls below the RO threshold forthe specified number of consecutive 200 .mu.sec windows (at 70). Thelatter may happen when RF holes occur in the normal RF footprint (i.e.,either gaps in the RF signal field at the tag antenna as a result of theCW reader alternating between being in and out of the response range ofthe tag, or nulls in the RF field such as where an RF lobe of thereader's transmitting antenna has sufficient energy to activate the tagbut objects somewhere in the lobe cause RF cancellation so that littleor no RF energy is present at the tag).

If the dominant mode is read-only (at 64), the backscattering FET willbe enabled if and when the RF level detected from an input signal viathe tag's antenna is above the RO threshold (at 70), which would resultin a backscatter response to the CW reader (at 72) in the form of acontinuous transmission of the same data repeatedly from the tag'smemory. The ASIC continuously scrolls the RO frame data whilesimultaneously allowing possible detection of a modulated RF inputsignal (at 62) from a modulating reader at the tag's antenna. Tagoperation continues as described above.

An alternative and perhaps more straightforward summary of the dual modetag's operation is illustrated by the state diagram of FIG. 4. Power-OnReset (POR) indicates a reset state in which the device has insufficientenergy (POR=1) to be booted up. The tag is essentially waiting forreceipt of an RF signal from a reader to cause the tag to boot up(POR=0). When it moves to the boot state, the tag commences to gatherthe configuration variables that are designated according to whether thedominant bit is set to dictate RO (DOM=1) or R/W (DOM=0) as its dominantoperating mode. If the RO mode (shown in FIG. 4 as ATA operation) isdominant, the tag remains in that state (Present State=1), which meansthat the backscatter FET 15 (FIG. 1) is disabled. That condition of thetag continues unless and until the level of a detected incoming RFsignal meets or exceeds the designated R/W RF threshold.

When the designated R/W RF threshold is met, the state machinerecognizes that it is encountering a modulating reader and the tagswitches from RO to the R/W state (Present State=0, shown in FIG. 4 asSeGo operation). The tag then cycles continuously in the R/W state untilthe criteria implemented by the detection rule indicate that theincoming signal level to the tag no longer meets the designatedthreshold (i.e., has either become weak or is no longer present), inwhich case the tag reverts to the RO state, or that a modulating commandis being received to cause the tag (Decode=1) to enter the DecodeCommand State.

In the latter event, the criteria implemented by the rule determinewhether the command is valid (e.g., frequency of occurrence of fallingedges within the predetermined period is sufficient) and, if a detectedcyclic redundancy code (CRC) is also correct, the tag executes thecommand and transmits in the proper response state for that command. If,however, the command is invalid, or the CRC is bad, the state machineenters the end state. And if the command was found to be valid and hasbeen properly executed by the tag to produce the mandated response mode,the completion of the command is followed by entry into the end state.In either case, the tag remains in the end state for up to 8 clockcycles, and unless a new command is received in the interim, the tagreturns to the R/W mode where it remains until the criteria of thedetection rule dictate otherwise, as indicated above.

The following examples illustrate the operation of an ASIC associatedwith a dual mode RFID tag according to several presently preferredembodiments of the invention, with certain specific conditions andconfigurations of the ASIC. In many of these examples, it is assumed inthe opening statement that only one reader (either CW or modulating) isinvolved in scanning the tag, but it will be understood that the pointin each example (other than Example 1) is that the dual mode tag of theinvention may come into the presence of either type of reader, and willrespond accordingly.

ASIC Operation, Example 1

Assumptions of prevailing conditions for this example: RFID tag readeris a Transcore model AI1200/AR2200 (CW); ASIC is configured for ROdominant (Dom. Bit=1), with values Min=3, Max=8, N=2, M=7.

Following power-up and boot sequence, ASIC sets tag operation in ROmode, and backscattering FET is enabled when RF input signal level isabove RO threshold. The RO data frame is scrolled 100 times (whichhappens only because the tag is configured in solely RO mode), and ASICthen opens a window for modulation detection. If no modulation detected,i.e., edges .gtoreq.3 and .ltoreq.8 did not occur in 2 consecutive 200.mu.sec windows, scrolling of RO data is continued for another 100frames before next window of opportunity for modulation detection isopened. But assumptions for this example have the RFID reader restrictedto CW, so the ASIC would not switch the tag from the RO operating mode.

ASIC Operation Example 2

Assumptions of prevailing conditions for this example: The reader is aTranscore model AI2110 (read/write capable, i.e., modulating); and theASIC is configured for RO dominant mode, with Min=3, Max=8, N=2, andM=7.

Following power-up and boot sequence, ASIC initiates RO mode; andbackscattering FET is enabled when RF level above RO threshold. RO dataframe is scrolled until detection of modulation. If no modulationdetected, i.e., edges .gtoreq.3 and .ltoreq.8 did not occur in 2consecutive 200 .mu.sec windows, scrolling continues. But if modulationis detected, tag's operating mode is switched to R/W. ASIC processes allR/W commands until no modulation detected during an IDLE state, i.e.,edges .gtoreq.3 not having occurred in (7.times.10=70) 200 .mu.secconsecutive windows. Tag then switched back to RO mode, and ASICcontinuously scrolls RO data frame. Operation continues in RO mode untilmodulation detection, at which point R/W mode may be reinstated ifapplicable conditions satisfied.

ASIC Operation Example 3

Assumptions of prevailing conditions for this example: RFID tag readeris Transcore model AI1200/AR2200 (CW); RFID tag with ASIC configured forRO dominant mode, but tag capable of both RO and R/W modes, Min=3,Max=8, N=2, M=7.

Following the power-up and boot sequence, ASIC moves the tag to RO mode,and backscattering FET enabled when RF level above RO threshold. TheASIC continuously scrolls the RO frame data and simultaneously allowsmodulation detection, which occurs if the R/W threshold is exceeded bythe average detected RF level and edges .gtoreq.3 and .ltoreq.8 tookplace in 2 consecutive 200 .mu.sec windows, and if not, scrolling of ROdata frame continues. If modulation detected by satisfying the aboverule, the tag is switched to R/W mode. And once in R/W mode, all R/Wcommands are processed by the ASIC until no modulation detected duringIDLE state, i.e., edges .ltoreq.3 occurring in (7.times.10-70)consecutive 200 .mu.sec windows, at which point tag is switched back toRO mode and scrolling of RO data.

ASIC Operation, Example 4

Assumptions of prevailing conditions for this example: Reader isTranscore model AI2110 (R/W capable, i.e., modulating); RFID tag's ASICconfigured in RO dominant mode (Dom. Bit=1), adapted for both RO andR/W, Min=3, Max=8, N=2, and M=7.

Following power activation and boot sequence, ASIC places tag in thedominant RO mode. Backscattering FET enabled and ASIC continuouslyscrolls RO frame data while simultaneously allowing possible detectionof modulated RF input signal. Such detection occurs if and when R/Wdetection rule has been met: R/W threshold exceeds average RF level andedges .gtoreq.3 and .ltoreq.8 within 2 consecutive 200 .mu.sec windows.If modulation not detected, ASIC continues scrolling data in RO mode.But if modulation detected, ASIC switches tag to R/W mode, and oncethere, ASIC processes all R/W commands. If no further modulation isdetected, ASIC would normally switch tag back to RO mode. But in thisexample, tag remains in R/W mode since it is assumed that RFID reader istransmitting a modulating signal.

ASIC Operation Example 5

Assumptions of prevailing conditions for this example: TranscoreAI1200/AR2200 reader (CW); ASIC configured for R/W dominant mode, R/WOnly, Min=3, Max=8, N=2, M=7.

Following power-up and boot sequence, ASIC moves tag to R/W mode in IDLEstate. Since reader is non-modulating, ASIC stays in IDLE state untilthe RF signal falls below R/W threshold or disappears altogether. Here,however, ASIC never switches tag to RO mode since ASIC configuration isset to R/W only.

ASIC Operation Example 6

Assumptions of prevailing conditions for this example: Transcore AI2110reader (R/W capable, i.e., modulating); ASIC configured with R/Wdominant mode, R/W Only, Min=3, Max=8, N=2, M=7.

Following power-up and boot sequence, ASIC switches tag to R/W mode inIDLE state. Detected RF level controls ability to receive commands andmust exceed R/W threshold. In that case, ASIC processes all R/W commandsuntil RF signal level falls below R/W threshold or disappearsaltogether. In this case, however, ASIC never switches tag to RO mode,since ASIC is configured for R/W only.

ASIC Operation Example 7

Assumptions of prevailing conditions for this example: TranscoreAI1200/AR2200 reader (CW only); ASIC configured for R/W dominant, R/Wand RO, Min=3, Max=8, N=2, M=7.

Following power-up and boot sequence, ASIC switches tag to R/W mode inIDLE state. Reader modulation can only be detected as previouslydescribed. If modulation detection mode is satisfied, ASIC continuallyprocesses R/W commands until no modulation detected. Thereafter, if the10.times.7=70 edges in 200 .mu.sec windows condition is satisfied, thetag is switched to RO mode and commences scrolling its RO data. Then,either of two conditions cited above can cause return to R/W mode. Butin this example, the ASIC switched the tag from R/W to RO mode, and noreturn to the R/W mode takes place until the RF level falls below the ROthreshold since the reader is assumed to be CW and thus, non-modulating.

ASIC Operation Example 8

Assumptions of prevailing conditions for this example: Transcore AI2110reader (R/W capable, i.e. modulating); ASIC configured for R/W dominantmode, R/W and RO, Min=3, Max=8, N=2, M=7.

Following power-up and boot sequence, ASIC moves tag to R/W mode in IDLEstate. Reader modulation detected if modulation detection rulesatisfied. If so, ASIC continually processes R/W commands untilmodulation detection ceases. Then, if specified conditions aresatisfied, ASIC switches to RO and continuously scrolls its data.Thereafter, either of the two aforementioned conditions will cause tagto return to R/W mode. In this example, ASIC responds to the reader ineither operating mode: R/W or RO.

The foregoing description and drawings should be considered asillustrative only of the principles of the invention. The invention maybe configured in a variety of ways and is not intended to be limited bythe preferred embodiments or methods. Numerous applications of theinvention will readily occur to those skilled in the art from aconsideration of the above description. Therefore, it is desired thatthe invention not be limited to the specific examples disclosed or theconstruction and operation shown and described. Rather, all suitablemodifications and equivalents may be resorted to.

For example, under certain circumstances it may be that a RFID tag isimplemented for more than two modes of operation. In the claims thatfollow, the term “multi-mode” is intended to include two or more modes,although dual mode is the preferred embodiment.

The invention claimed is:
 1. A factory-configurable RFIDtransponder-comprising: a receiver configured for receiving a continuouswave RF signal and a modulated RF signal a logic circuit configured tobe capable of detecting whether said receiver is receiving saidcontinuous wave RF signal or said modulated RF signal and to cause theRFID transponder to transmit a first response if said continuous wavesignal is detected and a second response if said modulated signal isdetected and wherein said logic circuit is further configured to becapable of receiving an input designating either one of said continuouswave RF signal and said modulated RF signal as a dominant mode and theother one of said continuous wave RF signal and said modulated RF signalas a recessive mode; and wherein said logic circuit is furtherconfigured to be capable of operating according to rules for transitionfrom said dominant mode to said recessive mode, wherein said rules canbe configured to be more or less stringent at the factory.
 2. The RFIDtransponder of claim 1 wherein said logic circuit is further configuredto be capable of implementing transition rules based on detection ofedges of an envelope of said RF signals.
 3. The RFID transponder ofclaim 2, wherein said transition rules are configured to be morestringent for detecting transition from said dominant mode to saidrecessive mode than for detecting a transition from said recessive modeto said dominant mode.
 4. An RFID system comprising: the RFIDtransponder of claim 1 and an RFID reader configured to produce saidcontinuous wave RF signal and to receive said first response from saidRFID transponder.
 5. An RFID system comprising: the RFID transponder ofclaim 1 and an RFID reader configured to produce said modulated RFsignal and to receive said second response from said RFID transponder.6. The factory configurable RFID transponder of claim 1, which isfurther configured to be field configurable and wherein said logiccircuit is further configured to allow said rules to configured to bemore or less stringent in the field.
 7. The factory configurable RFIDtransponder of claim 1, wherein said factory configured stringencysetting is a programmed configuration variable.
 8. The factoryconfigurable RFID transponder of claim 6, wherein said factory or fieldconfigured stringency setting is a programmed configuration variable. 9.An RFID system comprising: the RFID transponder of claim 6 and an RFIDreader configured to produce said continuous wave RF signal and toreceive said first response from said RFID transponder.
 10. An RFIDsystem comprising: the RFID transponder of claim 6 and an RFID readerconfigured to produce said modulated RF signal and to receive saidsecond response from said RFID transponder.
 11. An RFID systemcomprising: the RFID transponder of claim 7 and an RFID readerconfigured to produce said continuous wave RF signal and to receive saidfirst response from said RFID transponder.
 12. An RFID systemcomprising: the RFID transponder of claim 7 and an RFID readerconfigured to produce said modulated RF signal and to receive saidsecond response from said RFID transponder.
 13. An RFID systemcomprising: the RFID transponder of claim 8 and an RFID readerconfigured to produce said continuous wave RF signal and to receive saidfirst response from said RFID transponder.
 14. An RFID systemcomprising: the RFID transponder of claim 8 and an RFID readerconfigured to produce said modulated RF signal and to receive saidsecond response from said RFID transponder.